This invention relates to an ultrasonic imaging apparatus for forming a tomogram of a subject by applying ultrasonic waves to the subject and processing echo waves reflected from the subject.
An ultrasonic imaging apparatus is used in an ultrasonic diagnosis apparatus which applies ultrasonic waves to a patient, converts echo waves coming from the patient into electrical signals, processes the signals into image signals, and displays a tomogram on a CRT monitor, thereby helping doctors to make a diagnosis. The imaging apparatus has an ultrasonic transducer, an A/D converter, a buffer memory and a frame memory. The transducer is brought into contact with the patient's body, and applies an ultrasonic beam to the region of interest, thereby performing sector scanning on the region of interest. The sector scanning is achieved by ultrasonic steering of the beam. The transducer converts the echo waves obtained by the ultrasonic steering into echo signals. The A/D converter converts the echo signals to digital image signals. The image signals are stored in the buffer memory. The image signals, or image data, is transferred to the frame memory. The data is read from the frame memory and is converted to analog data by a D/A converter. The analog data is supplied to a CRT monitor, which displays a tomogram of the region of interest.
The conventional ultrasonic imaging apparatus samples the image signal one pixel after another, in the direction of the depth from the surface of the patient's body. Each sampled pixel signal is stored in the corresponding memory element of the frame memory. When the pixel data is stored in any memory element other than the corresponding one (for example, when black image data is stored in the memory element to store white image data), the frame image (i.e., tomogram) stored in the frame memory is displaced from the original tomographic image in the depth direction. In the known apparatus, the image data is interpolated but only in the horizontal direction. Hence, a phenomemon called "areasing" inevitably occurs, and the image data obtained by this horizontal data-interpolation results in a stepwise distortion of the tomogram.